Skype has decided to allow WiFi access for free at some select airports in the USA this year. In looking at the map many appear to be Boingo locations but there's no mention of Boingo's involvement. That means Skype is likely paying Boingo for the access it has previously secured for users who take advantage of what is best viewed as Pay As You Go access to airport WiFi.
For those who don't subscribe to any type of WiFi access plan, or don't want to pay for a daily, hourly or even per minute plan at an airport, Free is a pretty good price this holiday season. In the long run Skype is likely hoping to expose more people to the availability of the pay when you need it plan as this does nothing to burn off Skype credit in a users account.
The map of where you can find the free hotsports can be found here.